Bone constraint mask or bone constraint per action?
Constraints have an influence property that can turn it on/off - an influence of 0.0 means the constraint has no effect while at 1.0 it is fully applied.

You can animate the influence for each action, if you keyframe the influence on the first frame, it will remain that way for the entire action. Each action can have a different value keyed.

With the mouse over the influence, press I to add a keyframe, you can also  RMB on the value and choose Insert Keyframe.

You can also keyframe the influence throughout an action, for example turn it on to pick up the magazine and turn it off to drop it.